The greatest effect on performance, positive or negative, will occur where the ISA deviation, as absolute value, is greatest. Thus, the difference between the pressure altitude and density altitude will be the greatest as well.

For this question, examine each option and select the environmental conditions which give the greatest absolute value of ISA deviation.

Generally, the ISA deviation is: OAT - ISA temperature at the altitude given. Assuming that the ISA temperature lapse rate is 2°C per 1 000 ft, we get:

• ISA deviation with 42°C at Sea Level is: OAT - ISA temperature at SL = +42°C - 15°C = +27°C.
• ISA deviation with 20°C at 1 000 ft AMSL: OAT - ISA temperature at 1 000 ft = +20°C - 13°C = +7°C.
• ISA deviation with 3°C at 3 000 ft AMSL: OAT - ISA temperature at 3 000 ft = +3°C - 9°C = -6°C.
• ISA deviation with 8°C at 3 000 ft AMSL: OAT - ISA temperature at 3 000 ft = +8°C - 9°C = -1°C.

Therefore, the effect on performance will be the greatest with 42°C at Sea Level in a desert area, where the absolute value of ISA deviation (27°C) is greatest.

Since the ISA deviation is positive, +27°C, that means that the atmosphere is hotter than ISA and the density altitude will be much greater than the pressure altitude, resulting in a degraded performance (the most negative effect).

The most advantageous environmental conditions will be 3ºC at 3 000ft AMSL, which will give the greatest negative ISA deviation, -6ºC, thus the least density altitude.